Optical information processing device and recording medium

ABSTRACT

An optical information processing device is provided with a multi-wavelength light source that emits light of two or more different wavelengths, a filter portion that separates the light emitted from the multi-wavelength light source according to wavelength, and a condensing lens that focuses a plurality of lights separated by the filter portion on the same point for multi-wavelength recording.

TECHNICAL FIELD

The present invention relates to optical information processing devicesand recording media used in the fields of optical recording and opticalcommunications.

BACKGROUND ART

Conventional optical information processing devices carry out recordingand reproduction using optical recording media such as CD-R, CD-RW, andDVD-RAM. With these recording media, recording is carried out usinglight of a single wavelength and therefore recording is carried outusing, for example, changes in the recording medium's refractive indexdue to phase changes or the like. As for recording techniques, there aresingle-layer and double-layer recording techniques, and recordingcapacity is limited by the surface area of the recording medium.

In carrying out reproduction from a recording medium, a laser light isfocused on the recording medium from an external portion and minisculeindentations formed on the transparent recording medium, or changes inthe reflectivity of refractive index change portions, are read, and thusthe recorded information is read out.

On the other hand, in recording on the recording medium, writing iscarried out by focusing light to the recording medium and causing achange such as phase change, sublimation, or perforation due to the heatat the light-focused area. The above is a recording-reproduction methodbased on a laser light source of one photon/one wavelength.

Furthermore, the use of recording media in a volumetric direction(volumetric recording) in order to improve the capacity of recordingmedia is being investigated. For example, proposals and experimentalmanufacture have been carried out such as a technique in whichinformation is volumetrically recorded within the surface and depth(thickness) direction of a bulk-state recording medium and an opticaldisk of a construction having multilayer recording layers. However, whenthe refractive index of each layer is different in the case of recordinglayers having a multilayer construction, there is a tendency formultiple interference of the laser light to occur as well as a tendencyfor recording interference to occur between layers. Furthermore, as thenumber of recording layers increases, there is less light reflected fromthe layers distant from the light source, and therefore it becomesdifficult to obtain a sufficient S/N ratio.

Further still, when carrying out recording using single photonabsorption, the recording layers are made into multiple layers and inorder to absorb the laser light pertaining to the wavelength range forenabling recording, it is necessary for the recording power of the lightto be extremely large when recording a layer that is distant from thelight source. When the power of the laser light source for recording isincreased, there is a problem known as cross erasure by whichinformation recorded on a recording layer close to the light source isinadvertently erased when recording information on a recording layerthat is distant from light source.

Recording techniques based on two-photon absorption have been proposedas a way to solve these problems. A conventional recording method usingtwo-photon absorption is disclosed in Y. Kawata, Optics Letters, Vol.23, No. 10, pp. 756-758, 1998 for example. In this recording method, apulse light of a 762 nm wavelength at approximately 130 fs is used as alaser light for writing and information is recorded on a recordingmedium made of LiNbO₃ crystal. A refractive index distribution is formedin the crystal using absorption of 381 nm wavelength light withtwo-photon absorption of 762 nm wavelength light. LiNbO₃ is transparentwith respect to light of a 762 nm wavelength and is absorptive withrespect to light of a 381 nm wavelength. Two-photon absorption isproduced based on a nonlinear optical effect in the focused spot of thelight, which is absorbed for recording. The recording medium does notabsorb light until the light power in the vicinity of the focused spotin which two-photon absorption is produced reaches a high-density state,and therefore light is absorbed in recording areas only. For thisreason, the problems of absorption and cross erasure, which are problemsin volumetric recording, do not occur and high-density volumetricrecording becomes possible.

In conventional optical recording methods using a single wavelength andsingle photon, the recording density has a threshold value due to suchfactors as the wavelength of the light source and the NA of therecording focusing optical systems such as the condensing lens, andtherefore further increases in capacity are difficult. Furthermore,there are the problems of interlayer recording interference and crosserasure when recording on a multilayer recording medium and there is theproblem that there are limits to making recording layer multilayered andvolumetric recording.

Furthermore, in the technique of using two-photon absorption of a singlewavelength, recording is carried out with a 381 nm wavelength based ontwo-photon absorption using a light of a long wavelength (inconventional examples, a light of a 762 nm wavelength) as the recordinglight. However, when reading out the recorded bits (information), it isnecessary to have a light that has a wavelength other than thewavelength of the laser light for recording. This is because therecording density is reduced if the laser light for recording is usedfor reading since its wavelength is long.

Furthermore, there are techniques of recording using light of twodifferent wavelengths. When recording using a light source of twodifferent wavelengths, the light of the two wavelengths produced fromdifferent emission apertures must be focused to the same point. For thisreason, there is a problem in that complicated focusing and opticalsystems are required to solve issues such as correction of wavelengthdispersion, focal point control, and focal point control usingwavelength variation of the light source. Furthermore, a high outputfemtosecond laser with a peak power of several 100 W is required inorder to use two-photon absorption. A large-size light source isrequired for this, which is a problem in that application to consumerproducts is difficult.

DISCLOSURE OF INVENTION

The present invention has been devised to solve these issues, and it isan object thereof to provide an optical information processing devicecapable of carrying out multi-wavelength recording without complicatedcontrol and a recording medium on which information is opticallyrecorded.

An optical information processing device according to the presentinvention is provided with a multi-wavelength light source that emitslight of two or more different wavelengths, a filter portion thatseparates the light emitted from the multi-wavelength light sourceaccording to wavelength, and a condensing lens that focuses a pluralityof light separated by the filter portion on the same point formulti-wavelength recording.

Furthermore, a recording medium according to the present inventionrecords information using light, wherein the recording medium issubstantially transparent with respect to two lights of differentwavelengths, information is recorded by a change of an opticalcharacteristic only when the two lights are focused on the same point,and a wavelength of one light of the two lights is ½ a wavelength of theother light.

Furthermore, another recording medium according to the present inventionrecords information using light, wherein another recording medium issubstantially transparent with respect to two lights of differentwavelengths, information is recorded by a change of an opticalcharacteristic only when the two lights are focused on the same point,the recording medium having a characteristic of being absorptive withrespect to a sum frequency of the two lights, and a wavelength of thesum frequency being given by λ1×λ2/(λ1+λ2), where the wavelength of oneof the lights is given as λ1 and the wavelength of the other light isgiven as λ2.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a side view showing a structure of an optical informationprocessing device according to Embodiment 1.

FIG. 2A is a graph showing the transmission characteristics of afundamental wave in a filter portion of the optical informationprocessing device according to Embodiment 1.

FIG. 2B is a graph showing the transmission characteristics of a higherharmonic wave in the filter portion of the optical informationprocessing device according to Embodiment 1.

FIG. 3 is a side view showing a structure of another optical informationprocessing device according to Embodiment 1.

FIG. 4A is a graph showing the transmission characteristics of afundamental wave in a filter portion of another optical informationprocessing device according to Embodiment 1.

FIG. 4B is a graph showing the transmission characteristics of a higherharmonic wave in a filter portion of another optical informationprocessing device according to Embodiment 1.

FIG. 5 is a side view showing a structure of yet another opticalinformation processing device according to Embodiment 1.

FIG. 6A is a graph showing the transmission characteristics of afundamental wave in a filter portion of yet another optical informationprocessing device according to Embodiment 1.

FIG. 6B is a graph showing the transmission characteristics of a higherharmonic wave in a filter portion of yet another optical informationprocessing device according to Embodiment 1.

FIG. 7 is a side view showing a structure of an optical informationprocessing device according to Embodiment 2.

FIG. 8A is a graph showing the transmission characteristics of afundamental wave in the filter portion of the optical informationprocessing device according to Embodiment 2.

FIG. 8B is a graph showing the transmission characteristics of a higherharmonic wave in the filter portion of the optical informationprocessing device according to Embodiment 2.

BEST MODE FOR CARRYING OUT THE INVENTION

With an optical information processing device according to the presentembodiment, complicated controls or the like are not necessary, sincemulti-wavelength recording is carried out using a plurality of lightsemitted from the same light source and the plurality of lights can befocused easily on the same point. With this, recording can be carriedout with recording interference when recording on a multilayer recordingmedium or recording on a recording medium in a volumetric direction.

Furthermore, it is preferable that the condensing lens focuses theplurality of lights separated by the filter portion on the same pointvia respectively different optical paths. With this, recording can becarried out with recording interference when recording on a multilayerrecording medium or recording on a recording medium in a volumetricdirection.

Furthermore, it is preferable that a polarizing filter for controllingthe light of different wavelengths into light of respectively differentpolarizations further is provided. With this, recording can be carriedout in the recording medium on which recording is to be practiced basedon a polarized light direction of the light used.

Furthermore, it is preferable that the multi-wavelength light source isprovided with a coherent light source that emits a fundamental wave andan optical wavelength conversion element for converting a portion of thefundamental wave emitted from the coherent light source into a higherharmonic wave, and emits light of two different wavelengths of thefundamental wave and the higher harmonic wave. With this, recording canbe carried out using the fundamental wave and the higher harmonic wavewith multi-wavelength recording and the fundamental wave and the higherharmonic wave can be emitted from the same point.

Furthermore, the multi-wavelength light source may be constituted by asemiconductor laser.

Furthermore, it is preferable that the coherent light source is providedwith a function of varying a wavelength of the emitted fundamental wave,a conversion efficiency of the higher harmonic wave of the opticalwavelength conversion element is changed by changing the wavelength ofthe fundamental wave emitted from the coherent light source, and anoutput ratio of the fundamental wave and the higher harmonic waveemitted from the multi-wavelength light source is controlled. With this,the output ratio of the fundamental wave and the higher harmonic wavecan be controlled easily.

Furthermore, it is preferable that the filter portion is an opticalfilter having a transmission characteristic that is dependent on awavelength of a light for at least one of transmittance, diffractionefficiency, and polarization, wherein the characteristic is not uniformwithin a surface of the optical filter. With this, the plurality oflights emitted from the multi-wavelength light source can be focused onthe same point such that their optical paths do not overlap.

Furthermore, it is preferable that the filter portion is a ring-shapedband aperture filter, and a transmission characteristic of a light isdifferent in a ring-shaped band aperture portion of the ring-shaped bandaperture filter and a portion other than the ring-shaped band apertureportion. With this, the spot diameter of light that penetrates thering-shaped band aperture portion can be reduced further with superresolution. For this reason, when the spot diameters of the lights aredifferent, it is possible to unite these spot diameters.

Furthermore, it is preferable that the filter portion is a ring-shapedband aperture filter, and only the fundamental wave penetrates thering-shaped band aperture portion of the ring-shaped band aperturefilter, and only the higher harmonic wave penetrates the portion otherthan the ring-shaped band aperture portion of the filter. With this, thefocused spot of the fundamental wave, which is larger compared with thehigher harmonic wave, can be reduced to make it easier to unite thefocused spot size of the higher harmonic wave and the fundamental wave.

Furthermore, it is preferable that the plurality of lights separated bythe filter portion is focused on the same point in the recording medium,the recording medium comprises a material wherein at least one ofrefractive index, absorption coefficient, and fluorescencecharacteristic changes by focusing the plurality of lights separated bythe filter portion, and information is recorded by focusing on the samepoint in the recording medium. With this, information can be recorded inthe recording medium.

Furthermore, it is preferable that the recording medium is made from aplurality of recording layers.

Furthermore, it is preferable that the recording medium is made of asingle layer and locations in which the information is recorded aredistributed in a thickness direction. With this, the recording mediumcan be manufactured at low cost.

Furthermore, it is preferable that the recording medium comprises aphotochromic material.

Furthermore, it is preferable that the fundamental wave and the higherharmonic wave are focused on the same point in a recording medium, therecording medium is substantially transparent to the fundamental waveand the higher harmonic wave, and has a characteristic of beingabsorptive with respect to a sum frequency of the fundamental wave andthe higher harmonic wave, a wavelength of the sum frequency is given byλ1×λ2/(λ1+λ2), where the wavelength of one of the lights is λ1 and thewavelength of the other light is λ2, and information is recorded byfocusing on the same point in the recording medium. With this,information can be recorded in a recording medium based on two-photonabsorption using the fundamental wave and the higher harmonic wavethereof.

Furthermore, it is preferable that the recording medium is made from aplurality of recording layers.

Furthermore, it is preferable that the recording medium is made of asingle layer and locations in which the information is to be recordedare distributed in the thickness direction. With this, the recordingmedium can be manufactured at low cost.

Furthermore, it is preferable that the recording medium comprises aphotochromic material.

Furthermore, with a recording medium of the present embodiment,recording can be carried out using absorption of two wavelengths.Furthermore, with a recording medium of another embodiment, recordingcan be carried out using two-photon absorption.

Furthermore, it is preferable that the recording medium comprises aphotochromic material.

Furthermore, it is preferable that the recording medium has a multilayerstructure.

The following are descriptions of specific embodiments of the presentinvention.

An optical information processing device of the present embodiment emitsa fundamental wave and a higher harmonic wave from the same point ofemission using an optical wavelength conversion element, making it easyto focus these on the same point with a condensing lens as a focusingoptical system. It further is characterized in that the fundamental waveand the higher harmonic wave are controlled individually using anoptical filter having transmission characteristics that are differentwith respect to the fundamental wave and the higher harmonic wave, andhigh functionality is achieved in two-wavelength recording using thefundamental wave and the higher harmonic wave.

As for the optical recording method, an optical recording method usingtwo different wavelengths is suitable, such as optical recording basedon two-photon absorption using a non-linear optical effect, opticalrecording based on a reaction with two wavelengths using a photochromiceffect, multilayer recording such as volumetric holograms and volumetricrecording.

Embodiment 1

The following is a description of an optical information processingdevice according to an Embodiment 1 of the present invention withreference to the accompanying drawings. With an optical informationprocessing device according to Embodiment 1, optical recording with twowavelengths in which there is no recording interference betweenrecording layers is possible in the recording of information to arecording medium that has multilayer recording layers.

There are several ways of recording on a recording medium capable ofrecording information by irradiating two wavelengths onto the recordingmedium. For example, there is a method in which the molecular structureof the recording medium is changed by irradiating a light of onewavelength and then information is recorded in this state by irradiatinglight of a different wavelength. Furthermore, there is a method in whichrecording and fixing are carried out with light of two wavelengths.Furthermore, there is a method in which information is recorded by usingthe two-photon absorption of two wavelengths and levels excited by twowavelengths. In the case of such two-wavelength recording, multilayerrecording is difficult when the two lights travel on the same opticalpath. When light of two wavelengths passes along the same optical pathin the case of recording to a recording medium of a multilayerstructure, the light of the two wavelengths is irradiated onto recordinglayers other than the layer on which recording is being attempted. Forthis reason, exposure occurs on layers other than the layer on whichrecording is being attempted and interlayer recording interferenceoccurs. Hereupon, description will be given concerning an opticalinformation processing device that, by using a light source and awavelength conversion element, is capable of emitting light of twowavelengths (a higher harmonic wave and a fundamental wave) from thesame point of emission and focusing the light on layers to whichrecording is being attempted, with the higher harmonic wave and thefundamental wave traveling on different optical paths.

FIG. 1 is a side view showing a structure of an optical informationprocessing device according to Embodiment 1. The optical informationprocessing device of Embodiment 1 is provided with a multi-wavelengthlight source 10, a collimator lens 3 that collimates light emitted fromthe multi-wavelength light source, a ring-shaped band aperture filter 4a, which is a filter that separates the light emitted from themulti-wavelength light source 10 into light of different wavelengthsaccording to wavelength, and a condensing lens 5 that condenses theplurality of lights separated by the ring-shaped band aperture filter 4a to the same single point within the recording medium in order to carryout multi-wavelength recording.

The multi-wavelength light source 10 is provided with, for example, aDBR semiconductor laser 1, which is a coherent light source, and anoptical wavelength conversion element 2 for converting the wavelength oflight emitted from the DBR semiconductor laser 1. For example, afundamental wave of an 820 nm wavelength emitted from the DBRsemiconductor laser 1 is wavelength-converted to, for example, a higherharmonic wave of a 410 nm wavelength by the waveguide-type opticalwavelength conversion element 2.

It should be noted that the waveguide-type optical wavelength conversionelement 2 is constituted by, for example, an optical waveguide and aperiodic-shaped polarization reversal structure formed within theoptical waveguide. Light transmitted by the optical waveguide iswavelength-converted by a non-linear grating constituted by theperiodic-shaped polarization reversal structure to generate a higherharmonic wave. The generated higher harmonic wave has a wavelength of ½or ⅓ the wavelength of the incident light (fundamental wave). Astructure in which Mg-doped lithium niobate is used is a typical exampleof the waveguide-type optical wavelength conversion element 2. Theperiodic-shaped polarization reversal structure is formed as a singlecrystal of Mg-doped lithium niobate. In this case, when there is a cycleof approximately 2.8 μm, the fundamental wave of a 820 nm wavelength canbe converted into a higher harmonic wave of a 420 nm wavelength.

Furthermore, the conversion efficiency of the optical wavelengthconversion element 2 is, for example, approximately 50%. That is,approximately half of the fundamental wave emitted from the DBRsemiconductor laser 1 is converted into a higher harmonic wave 8 and isemitted at the same time as a fundamental wave 7 from the opticalwavelength conversion element 2. The guided wave modes of the emittedfundamental wave 7 and the higher harmonic wave 8 are both fundamentalmodes of TE00.

The ring-shaped band aperture filter 4 a, which is a filter portion, isa filter that has different transmission characteristics in the surfacetherein. Specifically, there are different transmission characteristicsat a ring-shaped band aperture portion 14 a and the inner circletherein. FIGS. 2A and 2B use the horizontal axis to indicate positioningalong the surface direction of the ring-shaped band aperture filter 4 aand show the transmittance thereof on the vertical axis. FIG. 2A is agraph showing the transmission characteristics of a fundamental wave 7(light of an 820 nm wavelength) with regard to the ring-shaped bandaperture filter 4 a, and FIG. 2B is a graph showing the transmissioncharacteristics of a higher harmonic wave 8 (light of a 410 nmwavelength) with regard to the ring-shaped band aperture filter 4 a.

As is evident from FIGS. 2A and 2B, the fundamental wave 7 penetratesthe ring-shaped band aperture portion 14 a, but the higher harmonic wave8 does not penetrate. Furthermore, contrary to this, the higher harmonicwave 8 penetrates the area inside the ring-shaped band aperture portion14 a, but the fundamental wave 7 does not penetrate. For this reason, asshown in FIG. 1, the fundamental wave 7 and the higher harmonic wave 8emitted from the multi-wavelength light source 10 are incident on thering-shaped band aperture filter 4 a via the collimator lens 3, and areseparated and emitted such that they respectively take different opticalpaths. The fundamental wave 7 and the higher harmonic wave 8 emittedfrom the ring-shaped band aperture filter 4 a take different opticalpaths and are incident on the condensing lens 5 without overlapping.Since it passes through the ring-shaped band aperture portion 14 a, thefundamental wave 7 causes a super resolution phenomenon and is focusableon a spot smaller than the diffraction limit. For this reason, it ispossible to reduce the diameter of the focused spot and carry outextremely high-density recording.

The condensing lens 5 is provided with color correction functionalityfor correcting differences in the wavelengths of the fundamental wave 7and the higher harmonic wave 8. Further still, by passing through thecondensing lens 5, the fundamental wave 7 and the higher harmonic wave 8are focused on the same focal point 9 in a recording medium 6. At thistime, the fundamental wave 7 and the higher harmonic wave 8 travel ondifferent optical paths until the focal point 9 in the recording medium6, and therefore they are both focused and coincide on the focal point 9without overlapping. Thus, two-wavelength recording is carried out atthe focal point 9. It should be noted that the recording medium 6 is astructure in which a recording layer 6 a and an intermediate layer 6 bare alternately layered.

Such a structure is effective as an optical system in which to carry outrecording and reproduction for a multilayer recording medium using lightof two wavelengths and enables optical information processing devicesuch as this to be achieved. With the optical information processingdevice of Embodiment 1, light having the different wavelengths of thefundamental wave 7 and the higher harmonic wave 8 travels until thefocal point 9 on different optical paths, and therefore it is possibleto prevent interlayer recording interference in the recording medium 6.The number of layers of the recording medium can be increased such thathigh-density recording is achieved. Furthermore, since the fundamentalwave 7 and the higher harmonic wave 8 are separated from a single lightand emitted from the same multi-wavelength light source 10, thefundamental wave 7 and the higher harmonic wave 8 can be controlledeasily.

Using the optical information processing device of Embodiment 1, writing(recording) of information was carried out for a recording mediumconstructed of an azobenzene polymer. A certain type of azobenzenepolymer is a material with which recording can be carried out using twowavelengths.

The multi-wavelength light source 10 emits light of an 820 nm wavelengthas the fundamental wave 7 and emits light of a 410 nm wavelength as thehigher harmonic wave 8, which is wavelength-converted. The azobenzenepolymer of which the recording medium 6 is constructed is absorptivewith respect to light of a wavelength in the vicinity of 440 nm. When ablue region light (in the vicinity of 440 nm wavelength) is irradiated,azobenzene polymer changes repetitively to a cis-trans-cis derivative.By irradiating a fundamental wave having polarized light orthogonal topolarized blue region light to the azobenzene polymer that repetitivelychanges, the trans is fixed and the molecular direction is aligned inone direction. At this time, the refractive index of the recordingmedium 6 changes, thus making recording possible.

For this reason, in addition to the ring-shaped band aperture filter 4a, it is preferable to arrange an optical filter (polarizing filter)making the polarizing light direction of the fundamental wave 7 and thehigher harmonic wave 8 orthogonal. The optical filter (polarizingfilter) may be inserted between the ring-shaped band aperture filter 4 aand the condensing lens 5. It should be noted that it is preferable toadd an optical filter (polarizing filter) or the like making thepolarizing light direction of the fundamental wave and the higherharmonic wave orthogonal, according to the recording characteristics ofthe recording medium.

In this way, it is possible to carry out recording by focusing thefundamental wave 7 and the higher harmonic wave 8 on the focal point 9in the recording medium 6. Furthermore, the recording density can beimproved by making the recording medium 6 a multilayer structure and itis possible to record information on 20 or more recording layers 6 awithout causing recording interference between the layers of therecording medium 6. Furthermore, by using the optical informationprocessing device of Embodiment 1, it becomes possible to record to arecording medium having a multilayer structure comprising a photochromicmaterial. The reason for this is explained below.

The recording interference that occurs between layers formed in therecording medium 6 is particularly remarkable in the case ofphotochromic materials in which recording is carried out using aphotochemical reaction. In the case of thermal recording, sincerecording does not occur without a temperature above a fixedtemperature, recording does not occur even when a low power light isrepetitively irradiated. However, in the case in which a photochemicalreaction is produced such as in a photochromic material, the irradiationof light in the photochromic material accumulates. For this reason, theintegral value of the amount of irradiation increases due to repetitiveirradiation even for a low power light, resulting in the recordingmedium 6 being exposed. Accordingly, the optical information processingdevice of Embodiment 1 is particularly effective when using therecording medium 6 having a multilayer structure comprising aphotochromic material.

The optical information processing device of Embodiment 1 demonstratesthe following effects.

The optical information processing device of Embodiment 1 is structuredsuch that light of two wavelengths is emitted from the same point.Conventionally, light of two wavelengths with different points ofemission are separated by the ring-shaped band aperture filter and theselights are focused onto the same point. However, the designing of theoptical system this involves is considerably difficult and lacksstability. Further still, fine adjustments for the focus and trackingoptical systems become complicated.

In contrast to this, with the optical information processing device ofEmbodiment 1 shown in FIG. 1, it is possible to emit the fundamentalwave 7 and the higher harmonic wave 8 of different wavelengths from thesame point of emission by providing the wave-guiding wavelengthconversion element 2. Accordingly, this should be configured such thatlight of two different wavelengths emitted from the same point of lightsource is focused on the same point. Furthermore, the condensing lens 5easily can focus the different wavelengths of the fundamental wave 7 andthe higher harmonic wave 8 on the same point by carrying out colorcorrection. Furthermore, being configured as a confocal optical system,an effect is demonstrated in which a stable optical system can be easilyconfigured even with regard to disturbances such as wavelengthfluctuation of the optical system and point of emission positioningdisplacement.

The optical information processing device of Embodiment 1 shown in FIG.1 is provided with the ring-shaped band aperture filter 4 a. Of thelight emitted from the multi-wavelength light source 10, the light thatpenetrates the ring-shaped band aperture portion 14 a is that of thefundamental wave 7, and therefore focusing based on super-resolution ofthe fundamental wave 7 is possible. When the fundamental wave 7 and thehigher harmonic wave 8 are focused on the same point, the focused spotof the fundamental wave 7 becomes larger since the wavelength of thefundamental wave 7 is longer than the wavelength of the higher harmonicwave 8. Optical recording is carried out when the focused spots of thefundamental wave 7 and the higher harmonic wave 8 are at a coincidencepoint. For this reason, portions of the focused spot of the fundamentalwave 7 that extend outward from the focused spot of the higher harmonicwave 8 and do not overlap the focused spot of the higher harmonic wave 8do not play a part in optical recording and therefore these portions ofthe fundamental wave 7 are to no purpose.

In the optical information processing device of Embodiment 1, thefundamental wave 7 passes through the ring-shaped band aperture portion14 a, and therefore is focused with super-resolution. For this reason,compared with an ordinary focused spot diameter, it is focusable to afocused spot diameter of 1/1.2. In this way, the focused spot diameterof the fundamental wave 7 becomes smaller and becomes substantially thesame size as the focused spot diameter of the higher harmonic wave 8.Because of this, there is the advantage that the higher harmonic wave 8overlaps portions in which the fundamental wave 7 is focused and opticalrecording can be carried out without wasting the power of thefundamental wave 7.

Furthermore, the areas of the recording medium 6 that undergo recordingare the areas in which the two wavelengths overlap and where the twowavelengths are focused. A factor of the focusing characteristics thatis particularly necessary is that the higher harmonic wave 8 has a shortwavelength. For this reason, it is preferable as a transmissioncharacteristic of the ring-shaped band aperture filter 4 a that thetransmission area of the fundamental wave 7 is set to an area displacedfrom the central vicinity of the ring-shaped band aperture filter 4 aand that the transmission area of the higher harmonic wave 8 is set tothe central vicinity.

It should be noted that, in Embodiment 1, a ring-shaped band aperturefilter 4 a is used whose transmission characteristics have adistribution based on the shape of the ring-shaped band aperture, butthe same effect is also achievable by providing the ring-shaped bandaperture portion of the filter with a different diffraction efficiency.

It also should be noted that the multi-wavelength light source 10 wasprovided with the coherent light source 1 and the optical wavelengthconversion element 2 and emitted the fundamental wave 7 and the higherharmonic wave 8, but instead of this, it is also possible to use asemiconductor laser that produces light of different wavelengths. Atwo-wavelength or three-wavelength laser that produces light ofdifferent wavelengths is achievable with a semiconductor laser formed bydeveloping different laser mediums on the same substrate. The positionsof the active layers of a semiconductor laser that produces light ofdifferent wavelengths is close to 10 μm or less. Furthermore, as in thecase of the multi-wavelength light source 10 provided with the coherentlight source 1 and the optical wavelength conversion element 2, it isalso possible to focus light onto the same focal point 9 using anoptical system (condensing lens 5) that uses chromatic aberration. Inparticular, light source miniaturization can be achieved and isbeneficial when a two-wavelength laser is used as the multi-wavelengthlight source.

The ring-shaped band aperture filter 4 a is used in the opticalinformation processing device shown in FIG. 1, but there is nolimitation to this in the present invention. Any optical filter issufficient as long as it can achieve separation for the fundamental wave7 and the higher harmonic wave 8. In this way it is possible to preventinterlayer recording interference in the recording medium and it is easyto record to a multilayer-structured recording medium. Furthermore,multi-layer configurations are achievable based on photon moderecording.

FIG. 3 is a side view showing a structure of another optical informationprocessing device according to the Embodiment 1. An optical filter 4 bis arranged instead of the ring-shaped band aperture filter 4 a in theinformation processing device of FIG. 3, but other than that it has thesame structure as the optical information processing device shown inFIG. 1.

The transmission characteristics of the optical filter 4 b in FIG. 3 areshown in FIGS. 4A and 4B. FIGS. 4A and 4B use the horizontal axis toindicate positioning along the surface direction of the optical filter 4b and show the transmittance thereof on the vertical axis. FIG. 4A is agraph showing the transmission characteristics of a fundamental wave(light of an 820 nm wavelength) with regard to the optical filter 4 b,and FIG. 4B is a graph showing the transmission characteristics of ahigher harmonic wave (light of a 410 nm wavelength) with regard to theoptical filter 4 b. As evident from FIGS. 4A and 4B, the fundamentalwave 7 penetrates a left-side half portion of the optical filter 4 b,but the higher harmonic wave does not penetrate. Furthermore, the higherharmonic wave penetrates a right-side half portion of the optical filter4 b, but the fundamental wave 7 does not penetrate.

For this reason, as shown in FIG. 3, the fundamental wave 7 and thehigher harmonic wave 8 emitted from the multi-wavelength light source 10are incident on the optical filter 4 b via the collimator lens 3, andare separated and emitted such that they respectively take differentoptical paths on the left and right. The fundamental wave 7 and thehigher harmonic wave 8 separated by the optical filter 4 b are incidenton the condensing lens 5 and travel on different optical paths to befocused on the same focal point 9, which is a recording location in therecording medium 6.

Furthermore, in order to separate the fundamental wave and the higherharmonic wave, it is also possible to separate by bringing about agrating effect using a filter that separates cyclically. Furthermore, itis also possible to use such filters as a filter that separatestwo-dimensionally into random areas, and a filter that separates intoseveral areas.

Furthermore, as shown in FIG. 5, instead of a multilayer structuredrecording medium, it is also possible to use a single-layer structuredrecording medium in which recording is carried out in the directions ofthe surface and the depth (thickness) of the recording medium by way ofvolumetric recording. FIG. 5 is a side view showing a structure of yetanother optical information processing device according to Embodiment 1.A point of difference between the optical information processing deviceshown in FIG. 5 and the optical information processing device shown inFIG. 1 is that instead of the multilayer structured recording medium 6(FIG. 1), a single-layer structured recording medium 16 (FIG. 5) isused. Other than that, the structures of FIG. 1 and FIG. 5 are the same.FIGS. 6A and 6B use the horizontal axis to indicate positioning withinthe surface of the ring-shaped band aperture filter 4 a and show thetransmittance of the penetrating light on the vertical axis. FIG. 6A isequivalent to FIG. 2A, and FIG. 6B is equivalent to FIG. 2A.

Even with a single-layer structured recording medium 16, if thefundamental wave 7 and the higher harmonic wave 8 are controlled so asto focus on the focal point 9 desired for recording, it is possible torecord information at the focal point 9 the same as with a multilayerstructured recording medium. The single-layer structured recordingmedium 16 has the effect of having low manufacturing costs.

It should be noted that the multi-wavelength light source 10 wasdescribed as emitting light of two different wavelengths, but it is alsopossible that it emits light of three or more different wavelengths.Furthermore, the filter portion that separates the plurality of lightsemitted from the multi-wavelength light source 10 should be an opticalportion having transmission characteristics that arewavelength-dependent for at least one of transmittance, diffractionefficiency, and polarization and that has transmission characteristicsthat are not uniform within the surface of the optical filter.

Furthermore, the recording medium 6 should comprise a material thatchanges with respect to at least one of refractive index, absorptioncoefficient, and fluorescence characteristics by being focused on by theplurality of lights separated by the filter portion.

Embodiment 2

The following is a description of an optical information processingdevice according to Embodiment 2 of the present invention with referenceto the accompanying drawings.

An object of the optical information processing device according toEmbodiment 2 is to achieve optical recording with two-photon absorptionusing a non-linear optical effect based on light of two differentwavelengths. Description will be given concerning the opticalinformation processing device according to Embodiment 2 using FIG. 7.

The optical information processing device shown in FIG. 7 is aconfiguration arranged with an optical filter 4 c instead of thering-shaped band aperture filter 4 a of the optical informationprocessing device shown in FIG. 1. Furthermore, unlike the recordingmedium 6 shown in FIG. 1, a recording medium 26 shown in FIG. 7 isstructured using a material that is recordable by two-photon absorptionand is structured with alternate layers of a recording layer 26 a and anintermediate layer 26 b. Other than that, the structure is the same asthat of the optical information processing device of FIG. 1.

The transmission characteristics of the optical filter 4 c in FIG. 7 areshown in FIGS. 8A and 8B. FIGS. 8A and 8B use the horizontal axis toindicate positioning along the surface direction of the optical filter 4c and show the transmittance thereof on the vertical axis. FIG. 8A is agraph showing the transmission characteristics of a fundamental wave 7(light of an 820 nm wavelength) with regard to the optical filter 4 c,and FIG. 8B is a graph showing the transmission characteristics of ahigher harmonic wave 8 (light of a 410 nm wavelength) with regard to theoptical filter 4 c. As is evident from FIGS. 8A and 8B, the opticalfilter 4 c has a gradient-type transmittance distribution with respectto the fundamental wave 7. Specifically, in the central portion of theoptical filter 4 c, the fundamental wave 7 has high transmittance, andthe transmittance becomes progressively lower from the center outwards.Furthermore, the entire surface of the optical filter 4 c is transparentto the higher harmonic wave 8.

The fundamental wave 7 and the higher harmonic wave 8 are emitted fromthe same wave-guiding wavelength conversion element 2, but since theirwavelengths are greatly different, the confinement within their opticalwaveguides are different. For this reason, the angles of divergence fromthe optical waveguides are greatly different, with the angle ofdivergence of the fundamental wave 7 being larger than the angle ofdivergence of the higher harmonic wave 8. Accordingly, when the emittedlight from the optical wavelength conversion element 2 is collimated bythe collimator lens 3, the surface area of the fundamental wave 7 isconsiderably larger than that of the higher harmonic wave 8, and thefocusing characteristics of the fundamental wave 7 and the higherharmonic wave 8 are likely to be different.

There is a method of inserting an optical filter that blocks theperipheral portions of the fundamental wave 7 to make uniform thefocusing characteristics of the fundamental wave 7 and the higherharmonic wave 8, and this can be carried out easily. Further still, inorder to use the power of the fundamental wave 7 effectively, it isnecessary to correct the cross section area of the beam of thefundamental wave 7, but it is preferable that the higher harmonic wave 8is not subjected to any effect at this time. For this reason, it ispreferable that an optical filter having different transmissioncharacteristics for the fundamental wave 7 and the higher harmonic wave8 within its surface is used as in the optical filter 4 c shown in thetransmittance distribution in the above-mentioned FIGS. 8A and 8B.

In FIG. 7, a portion of a fundamental wave of an 820 nm wavelengthemitted from the DBR semiconductor laser 1 as a coherent light source iswavelength-converted to a higher harmonic wave of a 410 nm wavelength bythe waveguide-type optical wavelength conversion element 2. In this way,the fundamental wave 7 and the higher harmonic wave 8 are emittedsimultaneously from the waveguide arranged at the optical wavelengthconversion element 2. The guided wave modes both are emitted at thefundamental mode of TE00.

The fundamental wave 7 and the higher harmonic wave 8 are incident onthe optical filter 4c via the collimator lens 3. The fundamental wave 7and the higher harmonic wave 8, whose angles of convergence arecontrolled by the optical filter 4 c, are incident on the condensinglens 5. The condensing lens 5 is provided with color correctionfunctionality for correcting differences in the wavelengths of thefundamental wave 7 and the higher harmonic wave 8, and the fundamentalwave 7 and the higher harmonic wave 8 are focused on the same point inthe recording medium 26. Since the focusing characteristics of thefundamental wave 7 and the higher harmonic wave 8 are made uniform bythe optical filter 4 c, these are focused on the same point in therecording medium 26.

A high-output pulse light can be obtained by pulse driving the DBRsemiconductor laser 1 that outputs the fundamental wave. For example, apulse width less than 20 to 30 picoseconds can be produced by using asemiconductor laser that has a supersaturated absorber. The pulse widthcan be reduced further by wavelength-converting this using a non-linearoptical effect. Furthermore, high-output laser output with a lead valueof several 100 mW is possible by pulse-driving the DBR semiconductorlaser 1, and high-efficiency SHG (second harmonic generation) output canbe obtained by wavelength-converting this using the wavelengthconversion element 2. In this way, the fundamental wave 7 and the higherharmonic wave 8 emitted from the wavelength conversion element 2 arefocused on the focal point 9, which is the same point in recordingmedium 26, and the power density of the fundamental wave 7 and thehigher harmonic wave 8 is increased at the focal point 9. From this,two-photon absorption is produced using a non-linear optical effect.

If the wavelength of the fundamental wave 7 is given as λ1 and thewavelength of the higher harmonic wave 8 as λ2, then two-photonabsorption occurs at the sum frequency of fundamental wave 7 and thehigher harmonic wave 8. The wavelength λ3 of the sum frequency becomes:λ3=λ1×λ2/(λ1+λ2). When second harmonic generation is used for the higherharmonic wave 8, λ2=λ½, and therefore λ3=λ⅓. For example, two-photonabsorption of a 273 nm wavelength is produced using two photons of the820 nm wavelength fundamental wave 7 and the 410 nm wavelength higherharmonic wave 8. With two-photon absorption such as this, there is noneed for the output of the multi-wavelength light source 10 to becomeexcessively large.

Optical recording using light of a 273 nm wavelength based on two-photonabsorption is carried out in the recording medium 26. A material usedfor the recording medium 26 should absorb almost no light with regard to820 nm wavelength light (fundamental wave) and 410 nm wavelength light(higher harmonic wave), but absorb light of a 273 nm wavelength andshould be a material whose refractive index or absorption coefficientchanges. From this, it is possible for the fundamental wave 7 and thehigher harmonic wave 8 to reach the recording layer 26 a in which thefocal point 9 exists, without being absorbed in areas other than thefocal point 9 of the recording medium 26.

It should be noted that even when the two wavelengths are not related bybeing in a fundamental wave and its harmonic wave relationship,two-photon absorption occurs at a wavelength λ6=λ4×λ5/(λ4+λ5), which isthe sum frequency of these two lights, where the two wavelengths arerespectively λ4 and λ5.

Two-photon absorption is produced at the focal point 9 in order toincrease the power density. However, the transmission characteristics ofthe fundamental wave 7 and the higher harmonic wave 8 (recording light)are maintained at any recording layer 26 a other than the focal point 9,and therefore there is no recording interference. That is, opticalrecording can be carried out by increasing the power density of thefundamental wave 7 and the higher harmonic wave 8 (recording light),even at a recording layer 26 a distantly positioned from the surface ofthe recording medium 26 in the thickness direction. High transmissioncharacteristics are maintained as they are in the recording layer 26 ain which optical recording is not carried out. From this, opticalrecording can be carried out in the recording medium 26 even when thenumber of layers of the recording layer 26 a is increased.

In the optical information processing device of Embodiment 2, thefundamental wave 7 and the higher harmonic wave 8 are fundamental modesemitted from the same multi-wavelength light source 10, and thereforeare emitted from the same point of emission. For this reason, thecondensing lens 5 is able to focus easily the fundamental wave 7 and thehigher harmonic wave 8 on the same point by merely correcting chromaticaberration.

Furthermore, the fundamental wave 7 and the higher harmonic wave 8always maintain a λ2=λ½ relationship. This relationship is alwaysmaintained even when there are fluctuations in the environmentaltemperature or wavelength fluctuation of the light source, and thereforedesigning for correction of chromatic aberration is easy, thus havingthe effect that two wavelengths can be focused stably on the same point.Furthermore, since there is a high degree of allowance in themanufacture and adjustment of the respective components, an effect isachieved in that production yields and assembly yields are high.

When reproducing the information recorded in the recording medium 26, ahigher harmonic wave of a 410 nm wavelength for example should be used.Since recording can be carried out with a wavelength equivalent to 273nm when recording using a non-linear optical effect, recording densitycan be increased. Furthermore, since reading is carried with a higherharmonic wave of 410 nm, a recording density equivalent to a wavelengthof 410 nm is possible in consideration of crosstalk and the like.Recording of extremely high density can be carried out.

Furthermore, in carrying out overwrite recording, since recording andreproduction are repeated and recorded information is repetitivelyerased and recorded while being verified, it is essential that recordinglight and reproduction light can be focused on the same focal point 9and that there can be rapid switching between recording andreproduction.

With the optical information processing device of Embodiment 2, thefundamental wave 7 and the higher harmonic wave 8 are emitted from thesame point. Furthermore, the fundamental wave 7 and the higher harmonicwave 8 are used for the recording light and the higher harmonic wave 8is used for the reproduction light. For this reason, switching betweenthe recording light and the reproduction light can be performed easily.Thus, overwrite recording is possible. However, in optical systems inwhich different light sources are used and the recording light and thereproduction light are respectively focused on the same point as inconventional optical information processing devices, it is necessary tosynchronize the respective light sources in rapid switchovers.Furthermore, complicated control and optical systems are necessary forcorrecting wavelength fluctuation and power fluctuation in therespective light sources and for correcting focus.

With an optical information processing device of Embodiment 2, the powerand the ratio thereof of the fundamental wave 7 and the higher harmonicwave 8, which are required for recording and reproduction, can becontrolled easily. The ratio of the fundamental wave 7 and the higherharmonic wave 8 supplied by the optical wavelength conversion element 2depends on the conversion efficiency, and the conversion efficiencydepends on the wavelength of the fundamental wave 7. For example, it ispossible to control the output wavelength of the DBR semiconductor laser1, which is a coherent light source, using an electrode arranged in theDBR portion. For this reason, by controlling the DBR wavelength of theDBR semiconductor laser 1, it is possible to control the efficiency ofconverting from the fundamental wave to the higher harmonic wave, thatis, it is possible to control the output ratio of the fundamental waveand the higher harmonic wave. In recording using two-photon absorption,it is necessary to control the output of the two light waves accurately,and this can be achieved easily with an optical information processingdevice of Embodiment 2.

In order to make low-power recording possible with a recording mediumusing two-photon absorption, it is preferable to achieve two-photonabsorption efficiency improvements by increasing the nonlinear opticalconstant. In order to do this in the recording medium 26 that isconstituted by a multilayer structure of the recording layers 26 a andthe intermediate layers 26 b, it is preferable to use a highly nonlinearmaterial for the intermediate layers 26 b. The thickness of therecording layers 26 a is at or below the submicron level, and givingconsideration to the beam waist of the recording light, it is possibleto achieve higher efficiency by generating a higher harmonic wave basedon a non-linear optical effect at the intermediate layers 26 b, outsidethe recording layers 26 a, and absorbing this at the recording layers 26a.

Furthermore, since the intermediate layers 26 b and the recording layers26 a can be designed separately, it is possible to use a highlynonlinear material for the intermediate layers 26 b and to use amaterial that has a high recording sensitivity to higher harmonic wavesfor the recording layers 26 a. This makes possible low power recording.For example, low power recording is possible by increasing thesensitivity using a photochromic material for the intermediate layer 26b.

Furthermore, the intermediate layers 26 b may have a multilayer filmstructure. To achieve wavelength conversion, a material of highnonlinearity is required in the vicinity (beam waist vicinity) of thesurface side (light source side) of the recording layers 26 a, but inareas other than this there is no particular requirement fornonlinearity. Accordingly, high speed recording becomes possible whenthe intermediate layers 26 b have a multilayer structure and are formedsuch that there is a layer of high nonlinearity on the upper surface ofthe recording layers 26 a and a layer of high thermal conductivity onthe lower side of the recording layers 26 a.

For an intermediate layer 26 b with high nonlinearity, it is possible touse, for example, LiNbO₃ or LiTaO₃, or an inorganic nonlinear materialsuch as KTP and KNbO₃, an amorphous layer of an organic nonlinearmaterial, or microcrystal materials of these. Further still, it is alsopossible to use a glass material into which these nonlinear materialshave been admixed, or a glass material into which a semiconductor highlynonlinear material has been doped, or the like. Low-power, multilayerrecording becomes possible by using a transparent highly nonlinearmaterial.

It should be noted that in the optical information processing device ofEmbodiment 2, second harmonic generation is used for the higher harmonicwave 8 produced by the optical wavelength conversion element 2, but itis also possible to use a light due to third harmonic generation, sumfrequency, difference frequency, and parametric generation, for example.

It also should be noted that in the optical information processingdevice of Embodiment 2, the recording medium 26 was described as arecording medium 26 of a multilayer structure, but a recording mediumbased on volumetric recording can be similarly used. In the case of arecording medium based on volumetric recording, since film depositionprocessing to form a multilayer structure is not required, there is thebenefit that the cost of the recording medium can be reduced.Furthermore, volumetric recording can be applied to recording such asvolume hologram and bit-by-bit recording.

Furthermore, instead of the optical filter 4 c, it is possible to usethe ring-shaped band aperture filter 4 a that is used in Embodiment 1.And it is also possible to use an optical filter having transmissioncharacteristics other than that. For example, various kinds of opticalsystems can be used that are provided with optical characteristics suchas polarization, transmittance, absorption, and diffraction, or surfacedistribution of these optical characteristics that are different for thefundamental wave 7 and the higher harmonic wave 8. For example, thepresent invention can be applied not only to optical recording, but alsoto retrieving signals when reading out or to focusing, tracking, andreferencing or the like.

For example, in the case of an optical information processing device inwhich recording is carried out on a hologram recording material, anorganic photosensitive material is used in the recording medium. Organicphotosensitive materials are sensitive to short wavelengths and have ahigh exposure sensitivity. When the recording light is used in theposition detection for adjusting the tracking or focus of the recordingposition, there is a problem in that recording cannot be carried outwhen exposure begins. For this reason, it is possible to solve suchproblems by using a structure in which position detection is achievedwith a fundamental wave and recording is achieved with a higher harmonicwave. The fundamental wave and the higher harmonic wave are emitted fromthe same point, and therefore position detection can be achieved easilysince it is easy to make the focal point consistent. To use thefundamental wave in position detection, the filter portion should use adiffraction grating that diffracts only the light of the fundamentalwave for example.

As described above, by applying light from a light source that emits thefundamental wave 7 and the higher harmonic wave 8 from the same locationto two-photon absorption, recording interference between layers of therecording layer 26 a can be reduced, and it is possible to narrow thespacing between recording layers, thus achieving improvements inrecording density. Furthermore, instead of a multilayer structure, it isalso possible to make the recording medium a single layer structure.

In Embodiments 1 and 2, it is preferable that a material is used for therecording media 6 and 26 that is substantially transparent to thefundamental wave 7 and the higher harmonic wave 8, and whose opticalcharacteristics change only when these two lights are focused on thesame point. It should be noted that the wavelength of the higherharmonic wave 8 is two times the wavelength of the fundamental wave 7.

Furthermore, for the recording medium, it is preferable that a materialis used that is substantially transparent to light of two differentwavelengths, and whose optical characteristics change only when thesetwo lights are focused on the same point. When the wavelength of thesetwo lights are given as λ1 and λ2, the wavelength of the light havingabsorption characteristics can be expressed: λ1×λ2/(λ1+λ2).

Furthermore, the recording medium may comprise a photochromic materialand may have a multilayer structure.

INDUSTRIAL APPLICABILITY

The optical information processing device of the present inventionoffers easy control of multi-wavelength light and is useful as anoptical recording device or the like.

Furthermore, the recording medium of the present invention is useful asa recording medium for optical recording.

1. An optical information processing device comprising: amulti-wavelength light source that emits light of two or more differentwavelengths, a filter portion that separates the light emitted from themulti-wavelength light source according to wavelength, and a condensinglens that focuses a plurality of lights separated by the filter portionon the same point for multi-wavelength recording.
 2. The opticalinformation processing device according to claim 1, wherein thecondensing lens focuses the plurality of lights separated by the filterportion on the same point via respectively different optical paths. 3.The optical information processing device according to claim 1, furthercomprising a polarizing filter for controlling the light of differentwavelengths into light of respectively different polarizations.
 4. Theoptical information processing device according to claim 1, wherein themulti-wavelength light source comprises a coherent light source thatemits a fundamental wave and an optical wavelength conversion elementfor converting a portion of the fundamental wave emitted from thecoherent light source into a higher harmonic wave, and emits light oftwo different wavelengths, which are the fundamental wave and the higherharmonic wave.
 5. The optical information processing device according toclaim 1, wherein the multi-wavelength light source comprises asemiconductor laser.
 6. The optical information processing deviceaccording to claim 4, wherein: the coherent light source is providedwith a function of varying a wavelength of the emitted fundamental wave,a conversion efficiency of the higher harmonic wave of the opticalwavelength conversion element is changed by changing the wavelength ofthe fundamental wave emitted from the coherent light source, and anoutput ratio of the fundamental wave and the higher harmonic waveemitted from the multi-wavelength light source is controlled.
 7. Theoptical information processing device according to claim 1, wherein thefilter portion is an optical filter having a transmission characteristicthat is dependent on a wavelength of light for at least one oftransmittance, diffraction efficiency, and polarization, wherein thecharacteristic is not uniform within a surface of the optical filter. 8.The optical information processing device according to claim 1, wherein:the filter portion is a ring-shaped band aperture filter, and atransmission characteristic of a light is different in a ring-shapedband aperture portion of the ring-shaped band aperture filter and aportion other than the ring-shaped band aperture portion.
 9. The opticalinformation processing device according to claim 4, wherein: the filterportion is a ring-shaped band aperture filter, and only the fundamentalwave penetrates the ring-shaped band aperture portion of the ring-shapedband aperture filter, and only the higher harmonic wave penetrates theportion other than the ring-shaped band aperture portion of the filter.10. The optical information processing device according to claim 1,wherein: a plurality of lights separated by the filter portion isfocused on the same point in the recording medium, the recording mediumcomprises a material wherein at least one of refractive index,absorption coefficient, and fluorescence characteristic changes byfocusing the plurality of lights separated by the filter portion, andinformation is recorded by focusing on the same point in the recordingmedium.
 11. The optical information processing device according to claim10, wherein the recording medium is made from a plurality of recordinglayers.
 12. The optical information processing device according to claim10, wherein the recording medium is made of a single layer and locationsin which the information is recorded are distributed in a thicknessdirection.
 13. The optical information processing device according toclaim 10, wherein the recording medium comprises a photochromicmaterial.
 14. The optical information processing device according toclaim 4, wherein: the fundamental wave and the higher harmonic wave arefocused on the same point in a recording medium, the recording medium issubstantially transparent to the fundamental wave and the higherharmonic wave, and has a characteristic of being absorptive with respectto a sum frequency of the fundamental wave and the higher harmonic wave,a wavelength of the sum frequency is given by λ1×λ2/(λ1+λ2) when thewavelength of the fundamental wave is λ1 and the wavelength of thehigher harmonic wave is λ2, and information is recorded by focusing onthe same point in the recording medium.
 15. The optical informationprocessing device according to claim 14, wherein the recording medium ismade from a plurality of recording layers.
 16. The optical informationprocessing device according to claim 14, wherein the recording medium ismade of a single layer and locations in which the information isrecorded are distributed in a thickness direction.
 17. The opticalinformation processing device according to claim 14, wherein therecording medium comprises a photochromic material.
 18. A recordingmedium that records information using light, wherein: the recordingmedium is substantially transparent with respect to two lights ofdifferent wavelengths, information is recorded by a change of an opticalcharacteristic only when the two lights are focused on the same point,and a wavelength of one light of the two lights is ½ a wavelength of theother light.
 19. The recording medium according to claim 18, wherein therecording medium comprises a photochromic material.
 20. The recordingmedium according to claim 18, wherein the recording medium has amultilayer structure.
 21. A recording medium that records informationusing light, wherein: the recording medium is substantially transparentwith respect to two lights of different wavelengths, information isrecorded by a change of an optical characteristic only when the twolights are focused on the same point, the recording medium has acharacteristic of being absorptive with respect to a sum frequency ofthe two lights, and a wavelength of the sum frequency is given byλ1×λ2/(λ1+λ2), where the wavelength of one of the lights is λ1 and thewavelength of the other light is λ2.
 22. The recording medium accordingto claim 21, wherein the recording medium comprises a photochromicmaterial.
 23. The recording medium according to claim 21, wherein therecording medium has a multilayer structure.